Multi-stage optical amplifier and broadband communication system

ABSTRACT

A multi-stage optical amplifier that has an optical fiber with a first length of amplifier fiber and a second length of amplifier fiber. The optical fiber is configured to be coupled to a signal source that produces at least a signal wavelength λ s  and a pump source that produces a pump wavelength λ p . Pump wavelength λ p  is less than signal wavelength λ s . Signal input, signal output and pump input ports are each coupled to the optical fiber. A first lossy member is coupled to the optical fiber and positioned between the first and second lengths of amplifier fiber. A pump shunt is coupled to the signal input port and the signal output port.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part of application Ser. No.09/471,753, filed on Dec. 23, 1999, which is fully incorporated hereinby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to multi-stage optical amplifiers, andmore particularly to broadband communication systems that include one ormore multi-stage optical amplifiers.

2. Description of the Related Art

The demand for bandwidth continues to grow exponentially on fiber-opticsuperhighways due to applications such as data communications and theinternet. Consequently, there is much effort at exploiting the bandwidthof optical fibers by using higher speeds per channel. Examples includetime-division multiplexed systems-and wavelength-division multiplexing(WDM).

Most fiber-optic networks currently deployed use standard single-modefiber or dispersion-shifted fiber (DSF). Standard fiber has a zerodispersion wavelength around 1310 nm, and the dispersion is primarilyresulting from the inherent glass dispersion. Currently, most of theterrestrial network in the US and the world is based on standard fiber.

With DSF, waveguide dispersion is used to shift the zero dispersionwavelength to longer wavelengths. A conventional DSF has a zerodispersion wavelength at 1550 nm, coinciding with the minimum loss in afused silica fiber. However, the zero dispersion wavelength can beshifted around by varying the amount of waveguide dispersion added. DSFis used exclusively in two countries, Japan and Italy, as well as in newlong-haul links.

The limiting factors for a fiber-optic transmission line include loss,dispersion and gain equalization. Loss refers to the fact that thesignal attenuates as it travels in a fiber due to intrinsic scattering,absorption and other extrinsic effects such as defects. Opticalamplifiers can be used to compensate for the loss. Dispersion means thatdifferent frequencies of light travel at different speeds, and it comesfrom both the material properties and waveguiding effects. When usingmulti-wavelength systems and due the non-uniformity of the gain withfrequency, gain equalization is required to even out the gain over thedifferent wavelength channels.

The typical solution to overcoming these limitations is to periodicallyplace in a transmission system elements to compensate for each of theseproblems. For example, a dispersion compensator can be used to cancelthe dispersion, an optical amplifier used to balance the loss and a gainequalization element used to flatten the gain. Examples of dispersioncompensators include chirped fiber gratings and dispersion compensatingfiber (DCF). Examples of optical amplifiers include erbium-doped fiberamplifiers (EDFAs), Raman amplifiers, and non-linear fiber amplifiers(NLFAs).

Another problem that arises in WDM systems is interaction or cross-talkbetween channels through non-linearities in the fiber. In particular,four-wave mixing (4WM) causes exchange of energy between differentwavelength channels, but 4WM only phase matches near the zero dispersionwavelength. Consequently, if a fiber link is made from conventional DSF,it is difficult to operate a WDM system from around 1540-1560 nm. Thisturns out to be quite unfortunate because typical EDFA's have gain from1535-1565 nm, and the more uniform gain band is near 1540-1560 nm. Asecond fiber nonlinearity that can be troublesome is modulationinstability (MI), which is 4WM where the fiber's nonlinearindex-of-refraction helps to phase match. However, MI only phase matcheswhen the dispersion is positive or in the so-called soliton regime.Therefore, MI can be avoided by operating at wavelengths shorter thanthe zero dispersion wavelength.

As the bandwidth utilization over individual fibers increases, thenumber of bands used for transmission increases. For WDM systems using anumber of bands, additional complexities arise due to interactionbetween and amplification in multi-band scenarios. In particular,particular system designs are needed for Raman amplification inmulti-band transmission systems. First, a new nonlinearity penaltyarises from the gain tilt from the Raman effect between channels. Thisarises because long wavelength channels tend to rob energy from theshort wavelength channels. Therefore, a means of minimizing the gaintilt on existing channels with the addition of new WDM channels isrequired.

To minimize both the effects of 4WM and Raman gain tilt, anothertechnical strategy is to use distributed Raman amplification. In a WDMsystem with multi-bands, a complexity arises from interaction betweenthe different pumps along the transmission line.

There is a need for greater bandwidth for broadband communicationsystems. A further need exists for broadband communication systems withreduced loss. Yet another need exists for broadband communicationsystems in the short wavelength region (S-band) covering the wavelengthrange of approximately 1430-1530 nm. Another need exists for broadbandcommunication systems with improved dispersion compensation.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provideimproved multi-stage optical amplifiers and broadband communicationsystems.

Another object of the present invention is to provide multi-stageoptical amplifiers and broadband communication systems with greaterbandwidth.

Yet another object of the present invention is to provide multi-stageoptical amplifiers and broadband communication systems in the S band.

A further object of the present invention is to provide multi-stageoptical amplifiers and broadband communication systems that use standardfiber and DSF with different zero dispersion wavelengths.

Another object of the present invention is to provide a multi-stageoptical amplifier and broadband communication system that combines the Cand S bands.

Yet another object of the present invention is to provide multi-stageoptical amplifiers and broadband communication systems that combine theC, S and L bands.

A further object of the present invention is to provide multi-stageoptical amplifiers and broadband communication systems with gain tiltcontrol

It is yet another object of the present invention to provide WDM systemsover DSF links by using the “violet” band in Raman amplifiers withdispersion compensating fiber to avoid nonlinearity limitations from 4WMand MI.

These and other objects of the present invention are achieved in amulti-stage optical amplifier that has an optical fiber including afirst length of amplifier fiber and a second length of amplifier fiber.The optical fiber is configured to be coupled to a signal source thatproduces at least a signal wavelength λ_(p), and a pump source thatproduces a pump wavelength λ_(p). Pump wavelength λ_(p) is less thansignal wavelength λ_(s). Signal input, signal output and pump inputports are each coupled to the optical fiber. A first lossy member iscoupled to the optical fiber and positioned between the first and secondlengths of amplifier fiber. A pump shunt is coupled to the signal inputport and the signal output port.

In another embodiment, the present invention is a broadbandcommunication system with a transmitter and a receiver. An optical fiberis coupled to the transmitter and receiver. The optical fiber includesat least a first Raman amplifier fiber and a second Raman amplifierfiber. The optical fiber is configured to be coupled to at least onesignal source that produces at least a signal wavelength λ_(s) and atleast two pump sources that collectively produce a pump beam ofwavelength λ_(p). Pump wavelength λ_(p) is less than signal wavelengthλ_(s). Signal input, signal output and a first pump input port are eachcoupled to the optical fiber. The first Raman amplifier fiber ispositioned between the signal input port and the pump input port. Thesecond Raman amplifier fiber is positioned between the pump input portand signal output port. A second pump input port is coupled to theoptical fiber and positioned between the second Raman amplifier fiberand the signal output port. A first lossy member is positioned betweenthe pump input port and the signal output port. The lossy member islossy in at least one direction so that passage of the pump radiation ofwavelength λ_(p) from the second to the first length of amplifier fiberis substantially blocked.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of one embodiment of a multi-stage opticalamplifier of the present invention that includes a pump shunt.

FIG. 2 illustrates that the cutoff wavelength of the fiber used with thepresent invention should be shorter than the pump and signalwavelengths.

FIG. 3 is a schematic diagram illustrating the inclusion of a dispersioncompensating element, a gain equalization element and an add/dropmultiplexer to the multi-stage optical amplifier of the presentinvention.

FIG. 4 is a schematic diagram of another embodiment of a multi-stageoptical amplifier of the present invention that includes two pumpshunts.

FIG. 5 is a schematic diagram of another embodiment of a multi-stageoptical amplifier of the present invention that includes a pump shuntand four lengths of amplifier fiber.

FIG. 6 is a schematic diagram of one embodiment of a multi-stage opticalamplifier of the present invention that includes a pump shunt and twopump sources.

FIG. 7 is a schematic diagram of one embodiment of a multi-stage opticalamplifier of the present invention that includes a pump shunt and acirculator.

FIG. 8(a) is a schematic diagram of another embodiment of a multi-stageoptical amplifier of the present invention that includes two lengths ofRaman amplifier fiber and two pump sources.

FIG. 8(b) is a schematic diagram of an embodiment of the presentinvention with a discrete and a distributed amplifier; where distributedamplification is added with only counter-propagating Raman pumps

FIG. 8(c) is a schematic diagram of an embodiment of the presentinvention similar to FIG. 8(b) in which mid-span access is not availablebut bi-directional pumping is allowed.

FIG. 9 is a schematic diagram of another embodiment of a multi-stageoptical amplifier of the present invention that includes three lengthsof Raman amplifier fiber and three pump sources.

FIG. 10 is a schematic diagram illustrating four pump source whoseoutputs are combined using wavelength and polarization multiplexing.

FIG. 11 is a schematic diagram illustrating eight pump source whoseoutputs are combined using wavelength and polarization multiplexing.

FIG. 12 is a schematic diagram illustrating that Brillouin threshold fora laser diode pump source can be minimized with the inclusion of aspectrum broadening device.

FIG. 13 is a schematic diagram of a broadband booster amplifierembodiment of the present invention.

FIG. 14 is a schematic diagram of a broadband pre-amplifier embodimentof the present invention.

FIG. 15 is a schematic diagram of one embodiment of a broadbandcommunication system of the present invention.

FIG. 16 is a schematic diagram of another embodiment of a broadbandcommunication system of the present invention.

FIG. 17 is a schematic diagram of another embodiment of a broadbandcommunication system of the present invention.

FIG. 18 is a schematic diagram of another embodiment of a broadbandcommunication system of the present invention.

FIG. 19 is a schematic diagram of another embodiment of a broadbandcommunication system of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

One embodiment of the present invention, as illustrated in FIG. 1, is amulti-stage optical amplifier 10 with an optical fiber 12 including afirst length of amplifier fiber 14 and a second length of amplifierfiber 16. Optical fiber 12 is configured to be coupled to a signalsource 18 that produces at least a signal wavelength λ_(s) and a pumpsource 20 that produces a pump wavelength λ_(p). Pump wavelength λ_(p)is less than signal wavelength λ_(s). Signal input port 22, signaloutput port 24 and pump input port 26 are each coupled to optical fiber12. A first lossy member 28 is coupled to optical fiber 12 andpositioned between the first and second lengths of amplifier fiber 14and 16 respectively. A pump shunt 30 is coupled to signal input port 22and signal output port 24. Optionally, a second lossy member 32 iscoupled to pump shunt 30. Pump shunt 30 can be an optical fiber that isintegral with optical fiber 12 or a separate optical fiber.

Pump beam λ_(p) propagates towards signal input port 22 from firstlength of amplifier fiber 14 and away from signal input port 22 tosecond length of amplifier fiber 16.

First and second lengths of amplifier fiber 14 and 16 each preferablyhave a length greater than or equal to 200 m. Pump wavelength λ_(p) ispreferably in the range of 1300 nm to 1530 nm, and the signal wavelengthcan be in the range of 1430 to 1530 nm. Suitable pump sources 20 includebut are not limited to laser diodes (LD's), solid state lasers,fiber-based cascaded Raman wavelength shifters, cladding pumped fiberlasers and the like.

First lossy member 28 can be an optical isolator, an add/dropmultiplexer. a gain equalization member, a dispersion compensationelement and the like. One or both of first and second lengths ofamplifier fiber 14 and 16 can be Raman amplifiers. Lossy elements 28 canalso be placed before and after first and second lengths of amplifierfiber 14 and 16 to prevent disturbance of amplifier performance fromspurious reflections from the transmission line. Additionally, a secondlossy element 32 can be inserted into pump shunt 30 to reduce themulti-path interference of the signal beam in amplifiers 12 and 14.

Additionally, one or both of first and second lengths of amplifier fiber14 and 16 can be implemented in dispersion compensating fiber (DCF). ADCF is a fiber whose zero dispersion point is shifted to wavelengthsmuch longer than 1500 nm using the waveguide dispersion property.Consequently, DCF tend to have a small affective core area andsignificant germanium doping in the core, both of which lead to anenhancement of the Raman gain coefficient. DCF's are generally addedperiodically to a high-speed transmission link to compensate for thedispersion accumulated in the line.

In one embodiment, multi-stage optical amplifier 10 operates in a violetband between 1430 and 1530 nm. Fiber 12 is a DSF with at least one fibernon-linearity effect and a zero dispersion wavelength. In thisembodiment, multi-stage optical amplifier 10 provides gain in the violetband sufficiently far from the zero dispersion wavelength to avoidnon-linearity effects.

First length of amplifier fiber 14 preferably has lower noise thansecond length of amplifier fiber 16. Second length of amplifier fiber 16has a higher gain than first length of amplifier fiber 14. In oneembodiment, first length of amplifier fiber 14 has an optical noisefigure of less than 8 dB, and second length of amplifier fiber 16 has again level of at least 5 dB.

One or more WDM couplers 34 are used to couple a pump path from thesignal input port 22 to the signal output port 24. WDM couplers 34 aredesigned to pass (couple over) the signal band while coupling over(passing) the pump beams. Exemplary WDM couplers 34 includefused-tapered fiber couplers, Mach-Zehnder couplers, thin-filmdielectric filters, bulk diachronic elements and the like.

Signal input port 22 inputs signal λ_(s) which is amplified throughRaman scattering when first and second lengths of amplifier fiber 14 and16 are Raman amplifiers. The dispersion and length of the first andsecond lengths of amplifier fiber 14 and 16 can be selected to be of thesame magnitude of dispersion-length product as the transmission link butof the opposite sign of dispersion. First and second lengths ofamplifier fiber 14 and 16 are preferably made single spatial mode forpump source 20 and signal wavelengths by making the cut-off wavelengthof the gain fiber shorter than the pump wavelength. In particular, thecut-off wavelength is the wavelength below which first and secondlengths of amplifier fiber 14 and 16 support more than one mode orbecomes multi-mode. If the pump or signal falls into the multi-moderegion, then additional noise arising from the beating between differentmodes may arise.

As shown in FIG. 2 the fiber cut-off wavelength should be shorter thanthe pump wavelength λ_(p). Pump wavelength λ_(p) is shorter than signalwavelength λ_(s). Multi-stage optical amplifier 10 is pumped so the netgain equals or exceeds the sum of losses in the transmission link andfirst and second lengths of amplifier fiber 14 and 16.

FIG. 3 illustrates that a dispersion compensating element 33, gainequalization element 29 or an add/drop multiplexer 31 can be includedand positioned between first and second lengths of amplifier fiber 14and 16.

FIG. 4 illustrates an embodiment of multi-stage optical amplifier 10with a third length of amplifier fiber 42. Second lossy member 32 ispositioned between second and third lengths of amplifier fiber 16 and42. A second pump shunt is coupled to second and third WDM couplers 46and 48. Additional lengths of amplifier fiber can also be included.

As illustrated in FIG. 5, multi-stage optical amplifier 10 can include athird and a fourth length of amplifier fiber 42 and 50, respectively. Inthis embodiment, third and fourth lengths of amplifier fiber 42 and 50are coupled to pump shunt 30. Second lossy member 32 is positionedbetween third and fourth lengths of amplifier fiber 42 and 50.

In another embodiment of multi-stage optical amplifier 10, multiple pumpsources are utilized. In FIG. 6, pump source 20 is positioned betweenfirst length of amplifier fiber 14 and first lossy member 28. A secondpump source 52 is positioned between second length of amplifier fiber 16and signal output port 24 and is coupled to a second pump input port 54.First pump source 20 produces a pump beam of wavelength λ_(p1) andsecond pump source 52 produces 52 a pump beam of wavelength λ_(p2).Wavelength λ_(p1) and wavelength λ_(p2) can be the same or different.Pump sources 20 and 44 collectively produce a pump beam of wavelengthλ_(p). Pump wavelength λ_(p) is less than a signal wavelength λ_(s).

In another embodiment, illustrated in FIG. 7, multi-stage amplifier 10includes one or more circulators 56 to provide isolation between thefirst and second lengths of amplifier fiber 14 and 16. Circulator 56also is useful as a means of dumping the remaining pump which can bereused elsewhere for monitoring purposes.

As illustrated in FIG. 8(a), multi-stage optical amplifier 10 can havean open loop configuration, In this embodiment, optical fiber 12 ispumped by a pump beam generated by pump sources 20 and 52 and first andsecond lengths of amplifier fiber 14 and 16 are each Raman amplifiers.Optical fiber 12 is preferably single spatial mode at both the signaland pump wavelengths. Again, wavelength λ_(p1) and wavelength λ_(p2) canbe the same or different. The pump beam has a wavelength shorter thanthe signal wavelengths. Pump sources 20 and 52 collectively produce apump beam of wavelength λ_(p). An amplified signal is then outputthrough signal output port 24. Pump sources 20 and 52 are coupled inthrough WDM couplers 34 and 58 which transmit signal wavelength λ_(s)but couple over the pump wavelength λ_(p). First lossy member 28 ispositioned between pump input port 26 and signal output port 24. In thisembodiment, the signal flows in a first direction and the pump beamflows in a reverse direction relative to the first direction. First andsecond lengths of amplifier fiber 14 and 16 are pumped in acounter-propagating manner. It may also be desirous to havebi-directional pumping in second length of amplifier fiber 16 toincrease the power amplifier gain without severely impacting the noisefigure of multi-stage optical amplifier 10.

Other elements, including but not limited dispersion compensatingelement 33, gain equalization element and add/drop multiplexer 31 may beincluded and positioned between first and second lengths of amplifierfiber 14 and 16.

In another embodiment, illustrated in FIGS. 8(b)-8(c), first length ofamplifier fiber 14 is a distributed Raman amplifier fiber and secondlength of amplifier fiber 16 is a discrete Raman amplifier fiber. Adistributed Raman amplifier fiber is an amplifier where at least somepart of the transmission link is pumped and involved in amplification.In this embodiment, first lossy member 28 is not positioned betweenfirst and second lengths of amplifier fiber 14 and 16. In FIG. 8(b)distributed amplification is added with only counter-propagating Ramanpumps. When access at a mid-point stage exists alternate band pumps areadded at different spatial points to minimize nonlinear interactionbetween pumps. In FIG. 8(c) mid-span access is not available butbi-directional pumping is allowed. The embodiment of FIG. 8(c) can beused where alternate band Raman pumps are launched in differentdirections in order to minimize interaction between pumps.

The open loop embodiment of multi-stage optical amplifier 10 can havethree or more lengths of amplifier fiber. Referring now to FIG. 9, anembodiment of multi-stage optical amplifier 10 is illustrated with thirdlength of amplifier fiber 42 coupled to a third pump source 60 which isturn is coupled to a third pump input port 62. WDM coupler 64 is coupledto third pump input port 62. Some or all of first, second and third pumpsources 20, 52 and 60 can be laser diode sources. Pump source 60produces a pump beam of wavelength λ_(p3). Wavelengths λ_(p1), λ_(p2)and λ_(p3) can be the same or different. Pump sources 20, 44 and 60collectively produce pump beam of wavelength λ_(p). An amplified signalis then output through signal output port 24.

As illustrated in FIGS. 10 and 11 each of pump source 20, 52 and 60 caninclude multiple pump sources whose outputs can be combined usingwavelength and polarization multiplexing. Multiple combination ratings66 and PBS's 68 can be utilized. Additionally, some or all of themultiple pump sources which comprise pump sources 20, 52 and 60 can belaser diodes.

Referring now to FIG. 12, a spectrum broadening device 70 can be coupledto each pump source 20, 52 and 60. This is particularly useful for laserdiode pump sources. Spectrum broadening device 70 broadens the spectrumwhile minimizing Brillouin threshold. Suitable spectrum broadeningdevices 70 include but are not limited to, (i)a grating that issufficiently broadband that can be chirped and cascade individualwavelengths, (ii) positioning a grating in a laser diode external cavityto cause appropriate line broadening and (iii) a dithering drive.Additionally pump pulsing can be used to broaden the spectrum.

The Brillouin threshold is reached when the following condition issatisfied:${\overset{\sim}{g}}_{B} = {{P_{0}^{LD} \cdot \frac{L_{eff}}{A_{eff}}} \leq 18}$

where

P₀ ^(LD)=power of laser diode$L_{eff} = {{\frac{1}{\propto} \cdot \lbrack {1 - \exp^{- {\propto \quad L}}} \rbrack}\quad {effective}\quad {pumping}\quad {length}}$

A_(eff)=effective area of fiber 12${\overset{\sim}{g}}_{B} = {\frac{\Delta \quad \gamma_{B}}{{\Delta \quad \gamma_{B}} + {\Delta \quad \gamma_{P}}} \cdot g_{B}}$${\overset{\sim}{g}}_{B} = {\frac{\Delta \quad \gamma_{B}}{{\Delta \quad \gamma_{B}} + {\Delta \quad \gamma_{P}}} \cdot g_{B}}$

Multi-stage optical amplifier 10 can be an in-line broadband amplifier,a booster amplifier, a broadband pre-amplifier and incorporated in anyvariety of different broadband communication systems. In anotherembodiment, illustrated in FIG. 13, the present invention is a broadbandbooster amplifier 72 that includes a multi-stage optical amplifier 10coupled to a transmitter 73. Transmitter 73 can include a WDM combiner74 and a plurality of transmitters 76. The plurality of transmitters 76transmit a plurality of wavelengths. The plurality of wavelengths mayinclude at least a first band of wavelengths and a second band ofwavelengths. With the present invention, a variety of differenttransmitters 76 can be utilized including but not limited to laserdiodes, tunable lasers, or broadband sources such as continuum sourcesor light-emitting diodes.

FIG. 14 illustrates a broadband pre-amplifier embodiment of the presentinvention. Broadband pre-amplifier 78 includes multi-stage opticalamplifier 10 coupled to a receiver 80. Receiver 80 can include a WDMsplitter 82 coupled to a plurality of receivers 84. Suitable receivers84 include but are not limited to germanium or InGaAs or InGaAsPdetectors followed by electronics well known to those skilled in theart.

In another embodiment, illustrated in FIG. 15, the present invention isa broadband communication system 86. In this embodiment, multi-stageoptical amplifier 10 is an in-line broadband amplifier. Multi-stageoptical amplifier 10 is coupled to one or more transmitters 73 and oneor more receivers 80.

FIG. 16 illustrates another embodiment of the present invention which isa broadband communication system 88 that includes multi-stage opticalamplifier 10 coupled to a broadband pre-amplifier 90. Multi-stageoptical amplifier 10 is coupled to one or more transmitters 73 andbroadband pre-amplifier 90 is coupled to one or more receivers 80.

FIG. 17 illustrates yet another embodiment of a broadband communicationsystem 92 with a broadband booster amplifier 94 coupled to multi-stageoptical amplifier 10. One or more transmitters 73 is coupled tobroadband booster amplifier 94. One or more receivers 80 is coupled tomulti-stage optical amplifier 10.

Another embodiment of a broadband communication system 96 is illustratedin FIG. 18. In this embodiment, an in-line amplifier 98 is coupled toreceiver 80 and to a transmitter 100. Transmitter includes multi-stageoptical amplifier 10 coupled to transmitter 73.

FIG. 19 illustrates another broadband communication system 102 of thepresent invention. Broadband communication system 102 includesmulti-stage optical amplifier 10 coupled to broadband booster amplifier94 and broadband pre-amplifier 90. Broadband booster amplifier 94 iscoupled to one or more transmitters 73. Broadband pre-amplifier 90 iscoupled to one or more receivers 80.

While embodiments of the invention have been illustrated and described,it is not intended that these embodiments illustrate and describe allpossible forms of the invention. Rather, the words used in thespecification are words of description rather than limitation, and it isunderstood that various changes may be made without departing from thespirit and scope of the invention.

What is claimed is:
 1. A multi-stage optical amplifier, comprising: anoptical fiber including a first length of Raman amplifier fiber and asecond length of Raman amplifier fiber, the optical fiber configured tobe coupled to a signal source that produces at least a signal wavelengthλ_(s) and a pump source that produces a pump wavelength λ_(p), whereinpump wavelength λ_(p) is less than signal wavelength λ_(s), the firstRaman amplifier fiber having an optical noise figure of less than 8 dBand lower noise than the second Raman amplifier fiber, and the secondRaman amplifier fiber having a gain level of at least 5 dB; a signalinput port coupled to the optical fiber; a signal output port coupled tothe optical fiber; a pump input port coupled to the optical fiber; afirst lossy member coupled to the optical fiber and positioned betweenthe first and second lengths of Raman amplifier fiber, the first lossymember being lossy in at least one direction; and a pump shunt coupledto the signal input port and the signal output port.
 2. The multi-stageoptical amplifier of claim 1, wherein the first and second lengths ofRaman amplifier fiber each have a length greater than or equal to 200 m.3. The multi-stage optical amplifier of claim 1, wherein pump radiationof wavelength λ_(p) is in the range of 1300 nm to 1530 nm.
 4. Themulti-stage optical amplifier of claim 1, wherein signal radiation ofwavelength λ_(s) is in the range of 1430 to 1530 nm.
 5. The multi-stageoptical amplifier of claim 1, wherein the first lossy member is anoptical isolator.
 6. The multi-stage optical amplifier of claim 1,wherein the first lossy member is an add/drop multiplexer.
 7. Themulti-stage optical amplifier of claim 1, wherein the first lossy memberis a gain equalization member.
 8. The multi-stage optical amplifier ofclaim 1, wherein the first lossy member is a dispersion compensationelement.
 9. The multi-stage optical amplifier of claim 1, wherein atleast one of the first and second Raman fiber amplifiers is a dispersioncompensating fiber.
 10. The multi-stage optical amplifier of claim 9,wherein the first and Raman fiber amplifiers are each dispersioncompensating fibers.
 11. The multi-stage optical amplifier of claim 1,wherein the second length of amplifier fiber has a higher gain than thefirst length of amplifier fiber.
 12. The multi-stage optical amplifierof claim 1, further comprising: at least one WDM coupler to couple apump path from the signal input port to the signal output port.
 13. Themulti-stage optical amplifier of claim 1, further comprising: a pumpsource coupled to the pump input port.
 14. The multi-stage opticalamplifier of claim 1, further comprising: at least one laser diode pumpsource coupled to the pump input port.
 15. The multi-stage opticalamplifier of claim 1, further comprising: a second lossy member coupledto the pump shunt.
 16. The multi-stage optical amplifier of claim 1,wherein the pump shunt includes an optical fiber.
 17. A broadbandbooster amplifier, comprising: a plurality of transmitters transmittinga plurality of wavelengths; a combiner coupled to the plurality oftransmitters; an optical fiber coupled to the combiner, the opticalfiber including a first length of Raman amplifier fiber and a secondlength of Raman amplifier fiber, the optical fiber configured to becoupled to a signal source and a pump source, the first Raman amplifierfiber having an optical noise figure of less than 8 dB and lower noisethan the second Raman amplifier fiber, and the second Raman amplifierfiber having a gain level of at least 5 dB; a signal input port coupledto the optical fiber; a signal output port coupled to the optical fiber;a pump input port coupled to the optical fiber; a first lossy membercoupled to the optical fiber and positioned between the first and secondlengths of Raman amplifier fiber, the first lossy member being lossy inat least one direction; and a pump shunt coupled to the signal inputport and the signal output port.
 18. A broadband pre-amplifier,comprising: an optical fiber including a first length of Raman amplifierfiber and a second length of Raman amplifier fiber, the optical fiberconfigured to be coupled to a signal source and a pump source, the firstRaman amplifier fiber having an optical noise figure of less than 8 dBand lower noise than the second Raman amplifier fiber, and the secondRaman amplifier fiber having a gain level of at least 5 dB; a signalinput port coupled to the optical fiber; a signal output port coupled tothe optical fiber; a pump input port coupled to the optical fiber; afirst lossy member coupled to the optical fiber and positioned betweenthe first and second lengths of Raman amplifier fiber, the first lossymember being lossy in at least one direction; a pump shunt coupled tothe signal input port and the signal output port; a splitter coupled tothe signal output port; and a plurality of receivers coupled to thesplitter.
 19. A broadband communication system, comprising: atransmitter; an optical fiber including a first length of Ramanamplifier fiber and a second length of Raman amplifier fiber, theoptical fiber configured to be coupled to a signal source and a pumpsource, the first Raman amplifier fiber having an optical noise figureof less than 8 dB and lower noise than the second Raman amplifier fiber,and the second Raman amplifier fiber having a gain level of at least 5dB; a signal input port coupled to the optical fiber; a signal outputport coupled to the optical fiber; a pump input port coupled to theoptical fiber; a first lossy member coupled to the optical fiber andpositioned between the first and second lengths of Raman amplifierfiber, the first lossy member being lossy in at least one direction; apump shunt coupled to the signal input port and the signal output port;and a receiver coupled to the optical fiber.
 20. The system of claim 19,wherein the first and lengths of Raman amplifier fiber each have alength greater than or equal to 200 m.
 21. The system of claim 19,further comprising: a pump source producing radiation of wavelengthλ_(p) in the range of 1300 nm to 1530 nm.
 22. The system of claim 19,further comprising: a signal source producing radiation of wavelengthλ_(s) in the range of 1430 to 1530 nm.
 23. The system of claim 19,wherein the first lossy member is an optical isolator.
 24. The system ofclaim 19, wherein the first lossy member is an add/drop multiplexer. 25.The system of claim 19, wherein the first lossy member is a gainequalization member.
 26. The system of claim 19, wherein the first lossymember is a dispersion compensation element.
 27. The system of claim 19,wherein the first length of amplifier fiber is a Raman amplifier. 28.The system of claim 19, wherein the second length of amplifier fiber isa Raman amplifier.
 29. The system of claim 19, wherein at least one ofthe first and second Raman amplifier fibers is a dispersion compensatingfiber.
 30. The system of claim 29, wherein the first and Raman amplifierfibers are each dispersion compensating fibers.
 31. The system of claim19, wherein the second length of amplifier fiber has a higher gain thanthe first length of amplifier fiber.
 32. The system of claim 19, furthercomprising: at least one WDM coupler to couple a pump path from thesignal input port to the signal output port.
 33. The system of claim 19,further comprising: a laser diode pump source coupled to the pump inputport.
 34. The system of claim 19, further comprising: a second lossymember coupled to the pump shunt.
 35. The system of claim 19, whereinthe pump shunt includes an optical fiber.
 36. A broadband communicationsystem, comprising: a transmitter; an optical fiber coupled to thetransmitter, the optical fiber including a first length of Ramanamplifier fiber and a second length of amplifier Raman amplifier fiber,the optical fiber configured to be coupled to a signal source and a pumpsource, the first Raman amplifier fiber having an optical noise figureof less than 8 dB and lower noise than the second Raman amplifier fiber,and the second Raman amplifier fiber having a gain level of at least 5dB; a signal input port coupled to the optical fiber; a signal outputport coupled to the optical fiber; a pump input port coupled to theoptical fiber;, a first lossy member coupled to the optical fiber andpositioned between the first and second lengths of Raman amplifierfiber, the first lossy member being lossy in at least one direction; apump shunt coupled to tie signal input port and the signal output port;at least one in-line broadband amplifier coupled to the optical fiber;and a receiver coupled to the in-line broadband amplifier.
 37. Thesystem of claim 36, wherein the in-line broadband amplifier comprises:an optical fiber including a first length of amplifier fiber and asecond length of amplifier fiber, the optical fiber configured to becoupled to a signal source and a pump source; a signal input portcoupled to the optical fiber; a signal output port coupled to theoptical fiber; a pump input port coupled to the optical fiber; a firstlossy member coupled to the optical fiber and positioned between thefirst and second lengths of amplifier fiber, the first lossy memberbeing lossy in at least one direction; and a pump shunt coupled to thesignal input port and the signal output port.
 38. A broadbandcommunication system, comprising: a transmitter; a broadband boosteramplifier; an optical fiber coupled to the broadband booster amplifier,the optical fiber including a first length of Raman amplifier fiber anda second length of Raman amplifier fiber, the optical fiber configuredto be coupled to a signal source and a pump source, the first Ramanamplifier fiber having an optical noise figure of less than 8 dB andlower noise than the second Raman amplifier fiber, and the second Ramanamplifier fiber having a gain level of at least 5 dB; a signal inputport coupled to the optical fiber; a signal output port coupled to theoptical fiber; a pump input port coupled to the optical fiber; a firstlossy member coupled to the optical fiber and positioned between thefirst and second lengths of Raman amplifier fiber, tie first lossymember being lossy in at least one direction; a pump shunt coupled tothe signal input port and the signal output port; and a receiver coupledto the optical fiber.
 39. The system of claim 38, wherein the broadbandbooster amplifier comprises: a plurality of transmitters transmitting aplurality of wavelengths; a combiner coupled to the plurality oftransmitters; an optical fiber coupled to the combiner, the opticalfiber including a first length of amplifier fiber and a second length ofamplifier fiber, the optical fiber configured to be coupled to a signalsource and a pump source; a signal input port coupled to the opticalfiber; a signal output port coupled to the optical fiber; a pump inputport coupled to the optical fiber; a first lossy member coupled to theoptical fiber and positioned between the first and second lengths ofamplifier fiber, the first lossy member being lossy in at least onedirection; and a pump shunt coupled to the signal input port and thesignal output port.
 40. A broadband communication system, comprising: atransmitter; an optical fiber coupled to the transmitter, the opticalfiber including a first length of Raman amplifier fiber and a secondlength of Raman amplifier fiber, the optical fiber configured to becoupled to a signal source and a pump source, the first Raman amplifierfiber having an optical noise figure of less than 8 dB and lower noisethan the second Raman fiber amplifier, and the second Raman fiberamplifier having a gain level of at least 5 dB; a signal input portcoupled to the optical fiber; a signal output port coupled to theoptical fiber; a pump input port coupled to the optical fiber; a firstlossy member coupled to the optical fiber and positioned between thefirst and second lengths of Raman amplifier fiber, the first lossymember being lossy in at least one direction; a pump shunt coupled tothe signal input port and the signal output port; a broadbandpre-amplifier coupled to the optical fiber; and a receiver coupled tothe broadband pre-amplifier.
 41. The system of claim 40, wherein thebroadband pre-amplifier comprises: an optical fiber including a firstlength of amplifier fiber and a second length of amplifier fiber, theoptical fiber configured to be coupled to a signal source and a pumpsource; a signal input port coupled to the optical fiber; a signaloutput port coupled to the optical fiber; a pump in put port coupled tothe optical fiber; a first lossy member coupled to the optical fiber andpositioned between the first and second lengths of amplifier fiber, thefirst lossy member being lossy in at least one direction; a pump shuntcoupled to the signal input port and the signal output port; a splittercoupled to the signal output port; and a plurality of receivers coupledto the splitter.
 42. A broadband communication system, comprising: atransmitter; a broadband booster amplifier coupled to the transmitter;an optical fiber coupled to the booster broadband amplifier, the opticalfiber including a first length of Raman amplifier fiber and a secondlength of Raman amplifier fiber, the optical fiber configured to becoupled to a signal source and a pump source, the first Raman amplifierfiber having an optical noise figure of less than 8 dB and lower noisethan the second Raman amplifier fiber, and the second Raman amplifierfiber having a gain level of at least 5 dB; a signal input port coupledto the optical fiber; a signal output port coupled to the optical fiber;a pump input port coupled to the optical fiber; a first lossy membercoupled to the optical fiber and positioned between the first and secondlengths of Raman amplifier fiber, the first lossy member being lossy inat least one direction; a pump shunt coupled to the signal input portand the signal output port; a broadband pre-amplifier coupled to theoptical fiber; and a receiver coupled to the broadband pre-amplifier.43. The system of claim 42, wherein the broadband booster amplifiercomprises: a plurality of transmitters transmitting a plurality ofwavelengths; a combiner coupled to the plurality of transmitters; anoptical fiber coupled to the combiner, the optical fiber including afirst length of amplifier fiber and a second length of amplifier fiber,the optical fiber configured to be coupled to a signal source and a pumpsource; a signal input port coupled to the optical fiber; a signaloutput port coupled to the optical fiber; a pump input port coupled tothe optical fiber; a first lossy member coupled to the optical fiber andpositioned between the first and second lengths of amplifier fiber, thefirst lossy member being lossy in at least one direction; and a pumpshunt coupled to the signal input port and the signal output port. 44.The system of claim 43, wherein the broadband pre-amplifier comprises:an optical fiber including a first length of amplifier fiber and asecond length of amplifier fiber, the optical fiber configured to becoupled to a signal source and a pump source; a signal input portcoupled to the optical fiber; a signal output port coupled to theoptical fiber; a pump input port coupled to the optical fiber; a firstlossy member coupled to the optical fiber and positioned between thefirst and second lengths of amplifier fiber, the first lossy memberbeing lossy in at least one direction; a pump shunt coupled to thesignal input port and the signal output port; a splitter coupled to thesignal output port; and a plurality of receivers coupled to thesplitter.